1. Field of the Invention
The present invention relates to a drive circuit for a speed automatically adjusted fan, particularly to a drive circuit, with which the speed of the fan can be properly adjusted depending on the ambient temperature.
2. Description of Related Art
A prior art of drive circuit for a fan, as shown in FIG. 1, is illustrated. The output terminal of a Hall IC is connected to the base pole of a transistor (Q1'). The collector of the transistor (Q1') is connected to a coil (V1') of the DC fan and then to the base pole of another transistor (Q2') through a resistor (R2'). The collector of the transistor (Q2') is connected to another coil (V2') of the fan. Both the emitters in the two transistors (Q1', Q2') are connected to the negative pole of the power supply. The two transistors (Q1', Q2') control the coils (V1', V2') of the fan, while the Hall IC sequentially controls the connection and the disconnection between the two transistors (Q1', Q2') to alternately magnetize of the coils (V1', V2') so as to rotate the fan. However, the circuit passing through the coils (V1', V2') is not possible to be changed so as to adjust the speed of the fan with respect to the ambient temperature once the coils and the power supply are fixedly arranged. It is a trend that the speed of the fan can be a variable with respect to the change of the ambient temperature. In addition to running under a maximum load condition for dissipating the heat, the fan is possible to generate noise either under a minimum load condition.
Therefore, based on regulations of the energy and the noise for the environmental protection, the problems involved in the conventional drive circuit for a fan have to be solved advantageously. In view of the drawbacks of the conventional drive circuit, the inventor has dedicated in research and design and has endeavored tests. Finally, a drive circuit for a speed automatically adjusted fan in accordance with the present invention is developed.